1. Field of the Invention
The present invention relates to an amplifying circuit for amplifying an input audio signal and the like and, more particularly, to an amplifier which reduces the power consumption so as to be suitable for portable personal stereos and the like.
2. Description of the Related Art
As an amplifying circuit for amplifying an input audio signal in a portable radio, a personal stereo system, an amplifying circuit such as that shown in FIG. 5 is conventionally used.
In FIG. 5, an amplifying circuit 1 is mainly composed of an amplifier A1. DC power is supplied to a terminal 5 of the amplifier A1 and a constant-current source 7 for determining the operating current of the amplifier A1 is connected to a terminal 4.
The signal input to a signal input terminal 2 is supplied to the non-inverting input of the amplifier A1.
A DC power source 8 is connected to a reference voltage terminal 6 and the DC power source 8 is connected to the inverting input of the amplifier A1 through a resistor R2.
The amplifier A1 amplifies the input signal supplied to the non-inverting input at an amplification level determined by the ratio of the resistance values of a resistor R3 and a resistor R2, and outputs the amplified signal to a capacitor C1 and a resistor R6 through a signal output terminal 3.
The output current of the constant-current source 7 connected to the terminal 4 is set at such a value that the amplifier A1 can drive a load at the maximum output.
Since the operating current which flows in the above-described conventional amplifying circuit is constant whether the input signal is a zero signal, a minute signal or a large signal, the amplifying circuit wastes power when a minute signal or no signal is input.